Immutable data structures give us peace of mind, but using them is challenging. How do you build an immutable list? Why would you use one? Join us and learn what makes a data structure “persistent”, the holy grail combination of immutability and performance. We’ll see not just how to use them, but also why they’re a good idea and how they work. Most importantly, we’ll see how these data structures are useful in real-life programming scenarios. Master this cornerstone of functional programming and learn the answer to the ultimate riddle: how do you change a list while leaving it unchanged?

https://www.reddit.com/r/ruby/comments/3vl1n0/rubyconf_2015_changing_the_unchangeable_by_brad/